Ken Bredemeier contributed to this report.
U.S. Defense Secretary Mark Esper says Turkish President Recep Tayyip Erdogan "bears full responsibility" for the resurgence of Islamic State, a growing humanitarian crisis, and possible war crimes.
This was the Pentagon's strongest condemnation so far of Turkey's military operation against Kurdish fighters in northern Syria.
Esper calls Turkey's attacks on the Kurds "unnecessary and impulsive." He says it has undermined what he calls the "successful" multinational mission to defeat Islamic State in Syria by allowing "many dangerous ISIS detainees" to flee detention camps that had been guarded by the Kurds.
Esper says U.S. relations with Turkey have been damaged. He says he plans to go to Brussels next week to press other NATO allies to slap sanctions on Turkey.
Turkish forces entered into northern Syria last week after U.S. President Donald Trump ordered the pull out of the approximately 1,000 U.S. forces in the area. They will be redeployed elsewhere in the Middle East to "monitor the situation," according to Trump.
The U.S. had been fighting side-by-side with the Kurds in Syria to defeat Islamic State. The extremists were just one rebel faction trying to overthrow the Syrian government.
Turkey regards the Kurdish Syrian Democratic Forces as a terrorist group aligned with Kurdish separatists inside Turkey.
Syrian Kurds say they feel forsaken by the United States. They also believe much of the Arab world and the U.N. Security Council are ignoring them.
But Esper says Turkey's "irresponsible" actions have created an unacceptable risk to U.S. forces in northern Syria, including the possibility of the U.S. getting "engulfed in a broader conflict."
Trump continued Monday to defend his decision to order the U.S. out of the area against strong criticism from both parties and European allies.
"Do people really think we should go to war with NATO Member Turkey?" Trump tweeted. "Never ending wars will end! The same people who got us into the Middle East mess are the people who most want to stay there!"
Trump said he is raising tariffs on Turkish steel imports and is prepared to impose sanctions on current and former Turkish officials. Trump also says he is stopping trade talks with Turkey.
"I am fully prepared to swiftly destroy Turkey's economy if Turkish leaders continue down this dangerous and destructive path," he said.
Democratic Speaker of the House Nancy Pelosi has called on the entire House to pass a resolution condemning Trump's decision to pull out of Syria. But she also agrees that Turkey must be condemned for its actions.
Republican Senate Majority Leader Mitch McConnell said he is "gravely concerned" about the Turkish offensive, contending it will jeopardize "years of hard-won progress" in destroying Islamic State.
McConnell said he would hold talks with Trump administration officials this week on how "to avoid a strategic calamity" in the region.
Syrian forces entered a town near the Turkish border Monday, a day after reaching an agreement with Syrian Kurds to move into the region in an attempt to counter the Turkish onslaught.
Syria's state-run SANA news agency reported Monday's troop movement in Tal Tamr, about 20 kilometers from the border, saying it was done to "confront the Turkish aggression" and was welcomed by the people there.
The fighting since the Turkish operation began nearly a week ago has killed dozens of civilians, observers say.
The U.S. State Department has condemned reports of pro-Turkish fighters executing civilians.